您可以在[Project Generation - Template](https://fastapi.tiangolo.com/zh/project-generation/)的模板中找到一个 FastAPI 项目中的 Alembic 示例。具体在[`alembic`代码目录中](https://github.com/tiangolo/full-stack-fastapi-postgresql/tree/master/src/backend/app/alembic/)。

### 创建依赖项

现在使用我们在`sql_app/database.py`文件中创建的`SessionLocal`来创建依赖项。

我们需要每个请求有一个独立的数据库会话/连接（`SessionLocal`），在所有请求中使用相同的会话，然后在请求完成后关闭它。

然后将为下一个请求创建一个新会话。

    !!! tip
        Prefer to use the `Annotated` version if possible.

    ```Python hl_lines="128-134"
    {!> ../../../docs_src/security/tutorial005.py!}
    ```

## Dependency tree and scopes

Let's review again this dependency tree and the scopes.

👆 💪 🔎 🖼 ⚗ FastAPI 🏗 📄 ⚪️➡️ [🏗 ⚡ - 📄](../project-generation.md){.internal-link target=_blank}. 🎯 <a href="https://github.com/tiangolo/full-stack-fastapi-postgresql/tree/master/src/backend/app/alembic/" class="external-link" target="_blank"> `alembic` 📁 ℹ 📟</a>.

### ✍ 🔗

🔜 ⚙️ `SessionLocal` 🎓 👥 ✍ `sql_app/database.py` 📁 ✍ 🔗.

Although the hierarchical dependency injection system is very simple to define and use, it's still very powerful.

You can define dependencies that in turn can define dependencies themselves.

In the end, a hierarchical tree of dependencies is built, and the **Dependency Injection** system takes care of solving all these dependencies for you (and their sub-dependencies) and providing (injecting) the results at each step.

```

mit einem JSON-Body, der etwa Folgendes enthält:

```JSON
{
    "description": "Payment celebration",
    "paid": true
}
```

und sie würde eine Response von dieser *externen API* mit einem JSON-Body wie dem folgenden erwarten:

```JSON
{
    "ok": true
}
```

!!! tip "Tipp"

```
https://www.external.org/events/invoices/2expen51ve
```

⏮️ 🎻 💪 ⚗ 🕳 💖:

```JSON
{
    "description": "Payment celebration",
    "paid": true
}
```

& ⚫️ 🔜 ⌛ 📨 ⚪️➡️ 👈 *🔢 🛠️* ⏮️ 🎻 💪 💖:

```JSON
{
    "ok": true
}
```

!!! tip
    👀 ❔ ⏲ 📛 ⚙️ 🔌 📛 📨 🔢 🔢 `callback_url` (`https://www.external.org/events`) & 🧾 `id` ⚪️➡️ 🔘 🎻 💪 (`2expen51ve`).

!!! info
    你还可以使用：

    * `response_model_exclude_defaults=True`
    * `response_model_exclude_none=True`

    参考 <a href="https://docs.pydantic.dev/latest/concepts/serialization/#modeldict" class="external-link" target="_blank">Pydantic 文档</a> 中对 `exclude_defaults` 和 `exclude_none` 的描述。

#### 默认值字段有实际值的数据

!!! info "情報"
    以下も使用することができます:

    * `response_model_exclude_defaults=True`
    * `response_model_exclude_none=True`

    `exclude_defaults`と`exclude_none`については、<a href="https://docs.pydantic.dev/latest/concepts/serialization/#modeldict" class="external-link" target="_blank">Pydanticのドキュメント</a>で説明されている通りです。

    * FastAPI provides more features on top of Starlette. Features that you almost always need when building APIs, like data validation and serialization. And by using it, you get automatic documentation for free (the automatic documentation doesn't even add overhead to running applications, it is generated on startup).

!!! info
    You can also use:

    * `response_model_exclude_defaults=True`
    * `response_model_exclude_none=True`

    as described in <a href="https://docs.pydantic.dev/latest/concepts/serialization/#modeldict" class="external-link" target="_blank">the Pydantic docs</a> for `exclude_defaults` and `exclude_none`.